A V-belt type continuously variable transmission is widely used in straddle type vehicles such as scooter type two-wheeled motor vehicles. A V-belt type continuously variable transmission includes a primary sheave arranged on a primary shaft and having the output of a power source such as an engine inputted thereto, and a secondary sheave arranged on a secondary shaft and having an output to a driving wheel taken therefrom. The grooves of the primary and secondary sheaves are varied in width. A V belt is looped around the sheaves. A groove width adjusting mechanism adjusts looping radiuses of the V belt with respect to the sheaves by changing groove widths of the sheaves to thereby adjust a speed change ratio between the sheaves continuously and variably.
Usually, each of the primary and secondary sheaves is constructed of a fixed flange and a moving flange which form a V-shaped groove between them. The moving flanges move freely in the axial direction of the primary or secondary shaft. The groove width adjusting mechanism adjusts a speed change ratio continuously and variably by moving the moving flange.
Conventional V-belt type continuously variable transmissions of this type include a transmission in which the moving flange of the primary sheave is moved to adjust a groove width by an electric motor. Since the moving flange can be moved in both a direction to narrow the groove width of the primary sheave (top side) and a direction to widen the groove width (low side), the groove width can be freely adjusted (for example, refer to Japanese Patent No. 3043061).
In a scooter type two-wheeled motor vehicle provided with a mechanism for electronically controlling a V-belt type automatic continuously variable transmission, a speed change ratio is automatically changed on a program (map) previously inputted with respect to a vehicle speed and the number of revolutions of the engine (accelerator position) without requiring the operation of a rider. For this reason, the rider can easily operate the vehicle and application of the automatic continuously variable transmission to various kinds of vehicles has been tried.
While a two-wheeled motor vehicle provided with an automatic continuously variable transmission for performing this kind of electronic control has the merit of facilitating a driving operation, it cannot exert an engine brake effectively at the time of returning the accelerator and hence cannot take advantage of the engine brake effectively for driving. Moreover, a two-wheeled motor vehicle provided with an automatic continuously variable transmission cannot perform a kick down based on the intention of a rider such as is employed in AT vehicles such as automobiles, and hence is slow in acceleration when passing other vehicles or when running on climbing roads.
To solve these problems, an automatic continuously variable transmission provided with a deceleration lever for inputting the intention of a driver, wherein the speed change ratio is set manually according to the position of the deceleration lever (for example, refer to Japanese Patent No. 2950957); an automatic continuously variable transmission capable of changing a speed change ratio forcibly; and an automatic continuously variable transmission provided with a switch for shifting down manually (for example, refer to JP-A-62-175228) are known. These automatic continuously variable transmissions can produce an arbitrary speed change ratio optionally and continuously by the intention of a rider and make it possible to apply an engine brake to driving and to perform shifting down in advance, thereby resolving the problem of being slow in acceleration.
However, consideration is not given to forcibly and manually shifting down the continuously variable transmission by the intention of a rider, and then returning the mode (“kick down mode”) to a normal mode. That is, while shifting down is performed as required based on the intention of the rider, the rider might desire shifting up following the shifting down to be automatically performed. In other words, if a straddle type vehicle capable of returning a kick down mode smoothly to a normal speed change mode can be developed, it would improve riding comfort.